Researchers are close to accurately simulating the ground motions of earthquakes to predict how these could impact infrastructure

A team of researchers from the Department of Energy’s (DOE’s) Lawrence Berkeley (Berkeley Lab) and Lawrence Livermore (LLNL) national laboratories, as well as from the University of California at Davis, have developed the first-ever end-to-end simulation code to precisely capture the geology and physics of regional earthquakes, and how the shaking impacts buildings. The code will take advantage of exascale supercomputers, the future supercomputers that will be 50 times faster than the US’s most powerful system today. Their work is part of the DOE’s Exascale Computing Project (ECP), a collaborative effort between the DOE’s Office of Science and National Nuclear Security Agency and was recently published in the Institute of Electrical and Electronics Engineers (IEEE) Computer Society’s Computers in Science and Engineering.

Up to now, the estimation of ground motions and their impact on structures was based on an empirical approach. For example, in order to predict how an earthquake would affect infrastructure in the San Francisco region, researchers might look at a past event that was of about the same magnitude â€” even if it had happened somewhere else â€” and use those observations to predict ground motion in the Bay Area. Based on empirical analysis of these simulations, they would estimate the impact on various buildings. â€œIt is no surprise that there are certain instances where this method doesnâ€™t work so well,â€ says David McCallen, who leads an ECP-supported effort called High Performance, Multidisciplinary Simulations for Regional Scale Seismic Hazard and Risk Assessments. â€œEvery single site is different â€” the geologic makeup may vary, faults may be oriented differently and so on. So, our approach is to apply geophysical research to supercomputer simulations and accurately model the underlying physics of these processes.â€

Buildings and structures respond differently to certain seismic wave frequencies. Large structures like skyscrapers, bridges, and highway overpasses are sensitive to low frequency shaking, whereas smaller structures like homes are more likely to be damaged by high frequency shaking, which ranges from 2 to 10 hertz and above. According to McCallen, the simulations of high frequency earthquakes are more computationally demanding and will require exascale computers.

The first-of-a-kind simulation

Using NERSCâ€™s Cori supercomputer, the researchers successfully simulated a 6.5 magnitude earthquake on Californiaâ€™s Hayward fault at 3-hertz in about 12 hours with 2,048 Knights Landing nodes. This simulation also captured the impact of ground movement on buildings within a 100-square kilometer radius of the rupture, as well as 30km underground. With future exascale systems, the researchers hope to run the same model at a 5-10 hertz resolution in approximately five hours or less. â€œDue to computing limitations, current geophysics simulations at the regional level typically resolve ground motions at 1-2 hertz (vibrations per second). Ultimately, weâ€™d like to have motion estimates on the order of 5-10 hertz to accurately capture the dynamic response for a wide range of infrastructure,â€ says McCallen. â€œWe know that the manner in which a fault ruptures is an important factor in determining how buildings react to the shaking, and because we donâ€™t know how the Hayward fault will rupture or the precise geology of the Bay Area, we need to run many simulations to explore different scenarios. Speeding up our simulations on exascale systems will allow us to do that.â€

â€œThe San Francisco Bay is an extremely hazardous area from a seismic standpoint and the Hayward fault is probably one of the most potentially risky faults in the country,â€ says McCallen. â€œWe chose to model this area because there is a lot of information about the geology here, so our models are reasonably well-constrained by real data. And, if we can accurately measure the risk and hazards in the Bay Area, itâ€™ll have a big impact.â€ He notes that the current seismic hazard assessment for Northern California identifies the Hayward Fault as the most likely to rupture with a magnitude 6.7 or greater event before 2044. Simulations of ground motions from large â€” magnitude 7.0 or more â€” earthquakes require domains in the order of 100-500 km and resolution on the order of about one to five meters, which translates into hundreds of billions of grid points. As the researchers aim to model even higher frequency motions between 5 to 10 hertz, they will need denser computational grids and finer time-steps, which will drive up computational demands. The only way to ultimately achieve these simulations is to exploit exascale computing, McCallen explains.

How it works

The tool under development by the project team employs a discretization technique that divides the Earth into billions of zones, and each zone is characterized by a set of geologic properties. Then, simulations calculate the surface motion for each zone. With an accurate understanding of surface motion in a given zone, researchers also get more precise estimates for how a building will be affected by shaking. The teamâ€™s most recent simulations at NERSC divided a 100-km x 100-km x 30-km region into 60 billion zones. By simulating 30 km beneath the rupture site, the team can capture how surface-layer geology affects ground movements and buildings.